Emmanouil Zoros

On the experimental validation of model-based dose calculation algorithms for 192Ir HDR brachytherapy treatment planning

There is an acknowledged need for the design and implementation of physical phantoms appropriate for the experimental validation of model-based dose calculation algorithms (MBDCA) introduced recently in 192Ir brachytherapy treatment planning systems (TPS), and this work investigates whether it can be met. A PMMA phantom was prepared to accommodate material inhomogeneities (air and Teflon), four plastic brachytherapy catheters, as well as 84 LiF TLD dosimeters (MTS-100M 1  ×  1  ×  1 mm3 microcubes), two radiochromic films (Gafchromic EBT3) and a plastic 3D dosimeter (PRESAGE). An irradiation plan consisting of 53 source dwell positions was prepared on phantom CT images using a commercially available TPS and taking into account the calibration dose range of each detector. Irradiation was performed using an 192Ir high dose rate (HDR) source. Dose to medium in medium, [Formula: see text], was calculated using the MBDCA option of the same TPS as well as Monte Carlo (MC) simulation with the MCNP code and a benchmarked methodology. Measured and calculated dose distributions were spatially registered and compared. The total standard (k  =  1) spatial uncertainties for TLD, film and PRESAGE were: 0.71, 1.58 and 2.55 mm. Corresponding percentage total dosimetric uncertainties were: 5.4-6.4, 2.5-6.4 and 4.85, owing mainly to the absorbed dose sensitivity correction and the relative energy dependence correction (position dependent) for TLD, the film sensitivity calibration (dose dependent) and the dependencies of PRESAGE sensitivity. Results imply a LiF over-response due to a relative intrinsic energy dependence between 192Ir and megavoltage calibration energies, and a dose rate dependence of PRESAGE sensitivity at low dose rates (<1 Gy min-1). Calculations were experimentally validated within uncertainties except for MBDCA results for points in the phantom periphery and dose levels  <20%. Experimental MBDCA validation is laborious, yet feasible. Further work is required for the full characterization of dosimeter response for 192Ir and the reduction of experimental uncertainties.

On the total system error of a robotic radiosurgery system: phantom measurements, clinical evaluation and long-term analysis

The total system error (TSE) of a CyberKnife® system was measured using two phantom-based methods and one patient-based method. The standard radiochromic film (RCF) end-to-end (E2E) test using an anthropomorphic head and neck phantom and isocentric treatment delivery was used with the 6Dskull, Fiducial and Xsight® spine (XST) tracking methods. More than 200 RCF-based E2E results covering the period from installation in 2006 until 2017 were analyzed with respect to tracking method, system hardware and software versions, secondary collimation system, and years since installation. An independent polymer gel E2E method was also applied, involving a 3D printed head phantom and multiple spherical target volumes widely distributed within the brain. Finally, the TSE was assessed by comparing the delineated target in the planning computed tomography images of a patient treated for a thalamic functional target with the radiation-induced lesion defined on the six-month follow-up magnetic resonance (MR) images. Statistical analysis of the RCF-based TSE results showed mean  ±  standard deviation values of 0.40  ±  0.18 mm, 0.40  ±  0.19 mm, and 0.55  ±  0.20 mm for the 6Dskull, Fiducial, and XST tracking methods, respectively. Polymer gel TSE values smaller than 0.66 mm were found for seven targets distributed within the brain, showing that the targeting accuracy of the system is sustained even for targets situated up to 80 mm away from the center of the skull. An average clinical TSE value of 0.87  ±  0.25 mm was also measured using the FSE T2 and FLAIR post-treatment MR image data. Analysis of the long-term RCF-based E2E tests showed no changes of TSE over time. This study is the first to report long-term (>10 years) analysis of TSE, TSE measurement for targets positioned at large distances from the virtual machine isocenter, or a clinical assessment of TSE for the CyberKnife system. All of these measurements demonstrate TSE consistently  <  1 mm.

On source models for (192)Ir HDR brachytherapy dosimetry using model based algorithms.

A source model is a prerequisite of all model based dose calculation algorithms. Besides direct simulation, the use of pre-calculated phase space files (phsp source models) and parameterized phsp source models has been proposed for Monte Carlo (MC) to promote efficiency and ease of implementation in obtaining photon energy, position and direction. In this work, a phsp file for a generic (192)Ir source design (Ballester et al 2015) is obtained from MC simulation. This is used to configure a parameterized phsp source model comprising appropriate probability density functions (PDFs) and a sampling procedure. According to phsp data analysis 15.6% of the generated photons are absorbed within the source, and 90.4% of the emergent photons are primary. The PDFs for sampling photon energy and direction relative to the source long axis, depend on the position of photon emergence. Photons emerge mainly from the cylindrical source surface with a constant probability over  ±0.1 cm from the center of the 0.35 cm long source core, and only 1.7% and 0.2% emerge from the source tip and drive wire, respectively. Based on these findings, an analytical parameterized source model is prepared for the calculation of the PDFs from data of source geometry and materials, without the need for a phsp file. The PDFs from the analytical parameterized source model are in close agreement with those employed in the parameterized phsp source model. This agreement prompted the proposal of a purely analytical source model based on isotropic emission of photons generated homogeneously within the source core with energy sampled from the (192)Ir spectrum, and the assignment of a weight according to attenuation within the source. Comparison of single source dosimetry data obtained from detailed MC simulation and the proposed analytical source model show agreement better than 2% except for points lying close to the source longitudinal axis.

TH-AB-BRA-04: A Physical Phantom for Experimental Commissioning and Performance Testing of 192Ir MBDCAs

Purpose: To present a phantom-based methodology for the experimental commissioning and performance testing of model-based dose calculation algorithms (MBDCAs), which have been recently introduced in 192Ir HDR brachytherapy treatment planning systems (TPSs). Methods: The phantom was constructed from PMMA slabs properly machined to accommodate material and density inhomogeneity inserts, as well as TLD detectors (TLD-100, 1×1×1 mm3), radiochromic films (Gafchromic EBT-3) and a cylindrical Presage dosimeter (d=6cm, h=8cm). The spatial arrangement of the different dosimeters within the phantom permitted measurements at regions of scatter conditions departing from TG43 assumptions (e.g., at phantom boundary) and/or close to the inhomogeneity inserts. The phantom was CT-imaged and a multiple 192 Ir source position treatment plan was prepared using the Oncentra Brachy v4.5 TPS. Dose calculations of the Oncentra-ACE MBDCA were exported in DICOM-RT for further evaluation. The plan was delivered using a microSelectron v.2 192 Ir source and 4 plastic catheters embedded in PMMA slabs. The treatment plan data were subsequently imported into an in-house developed software tool (BrachyGuide) used to obtain corresponding Monte Carlo (MC) simulation dosimetry results with the MCNP code. Detectors’ measurements were compared to both MBDCA- and MC-calculated results in terms of absolute point dose differences, 2D and 3D relative dose and gamma index distributions. Results: Experimental dosimetry results and MC calculations were found in agreement within uncertainties. The corresponding dosimetry comparison between detector measurements and ACE-calculations showed also a good agreement which deteriorated with increasing distance from the implant due, mainly, to MBDCA assumptions and optimization settings. The latter was highlighted in the comparison of the 3D dose distribution measured by the Presage dosimeter to corresponding ACE calculations. Conclusion: The proposed phantom/methodology can be used for both commissioning and quality assurance of MBDCA-based TPSs, as well as for benchmarking MC-calculated reference dose distributions. Research co-financed by the ESF and Greek funds through the Operational Program Education and Lifelong Learning Investing in Knowledge Society of the NSRF. Research Funding Program: Aristeia. Nucletron, an Elekta company (Veenendaal, The Netherlands) is gratefully acknowledged for providing Oncentra Brachy v4.5 for research purposes.